1. Field of the Invention
The present invention generally relates to the connection of electrical terminals to plastic panels having electrically conductive grids, traces or circuits thereon, hereinafter just “grids”. More specifically, the present invention relates to the attachment of electrical terminals to an electrical grid applied to a plastic substrate of a plastic window system via film insert molding in order to provide a functional capability to the window system, such as defrosting and defogging capabilities.
2. Description of the Related Technology
Electrically conductive grids have long been used for providing functional capabilities to window systems, such as the defrosting and defogging of windows, particularly the backlights of automobiles. In an electrically heated backlight for example, an electrically conductive heating grid is typically located toward either the interior or exterior side of the window. The heating grid also typically includes a pair of opposed busbars, between which a series of grid lines extend. During the passing of electric current through the heating grid, the resistance of the grid lines results in the generation of heat, and this heat dissipates across the window, subsequently defrosting or defogging of the window. In order to provide electricity to the heating grid, the heating grid is coupled to the electrical system of the automotive vehicle.
To achieve coupling of the automotive vehicle's electrical system to the heating grid, the busbars of the heating grid have been formed with foil tabs that extend beyond the edges of the window. Wire harness terminals, from the vehicle's electrical system, engage the tabs. The terminals can be of a variety of constructions, but often include a spring metal contact, encased within a housing. When the housing is attached to a tab, the contact is biased into contact with the busbar. In an alternate construction, bonding pads are integrally formed or attached to the busbars and terminals from the vehicle's electrical system are soldered to the bonding pads.
In another alternate construction, the heating grid is applied to the substrate of the window by way of a film insert molding (FIM), which is sometimes also referred to as in-mold decorating (IMD). It is to these techniques that the present invention is particularly directed. With FIM, the heating grid is printed onto a flexible, thin plastic sheet, herein referred to as a film. The film is shaped so as to correspond to the shape of a cavity surface of the mold in which the plastic panel is to be formed. The film is placed in the mold and on the corresponding surface of the mold cavity such that the heating grid is toward the interior of the mold cavity. Thereafter, the mold is closed and the plastic resin for the substrate of the panel is injected into the mold, back filling the film and forming the panel with the plastic film adhered thereto. As a result, the heating grid is encapsulated in the panel between the film and the substrate. Once the panel is sufficiently cooled, the mold is opened and the panel, with the film and integrated heating grid, removed from the mold.
Each of the above constructions has its known problems and limitations. Illustrative of the limitations of the spring contacts is that over the life of the vehicle, the spring contacts may become loose, due to fatigue and/or vibration, resulting in a non-working or a poorly working heating grid. With regard to the limitations of a pad bonding construction, the application of too much or too little solder weakens the joint between the terminals and the bonding pad, which may result in the terminal being easily dislodged from the bonding pad itself. Due to the low glass transition temperature of plastics, traditional high temperature solder cannot be used to make robust connections to the busbars. Unfortunately, the commercially available low temperature solders, and even, electrically conductive adhesives, have unacceptable bonding strengths and or reliability.
Regarding heating grids applied via FIM, after molding, the heating grid are encapsulated between the film and plastic substrate of the panel. In order for the heating grid to function, the heating grid must connect to the electrical system of the vehicle, and more particularly, with an electrical terminal of that system. In view of the above, it is apparent that improved connection constructions for attaching terminals to the busbars of FIM conductive grids of plastic window systems are required.